I. Conversion of Biomass to Chemical Feedstocks and Hydrocarbon Fuels
Vegetation biomass (hereinafter "biomass") includes all of the plants, crops and trees found within a given habitat. Cellulose and hemicellulose, the chief components of biomass, are renewable resources of energy and chemical feedstocks (Technology Report, Chem. & Eng. News, Feb. 23, 1976, pp. 24; Goldstein, I. S., Science 189:847 (1975)). Although biomass may be converted into gas or liquid fuels by fermentation or by pyrolysis at high temperature and pressure, these processes have significant drawbacks.
Fermentations suffer from poor efficiency. Processes resulting in the production of methane conserve only about 60% of the fixed energy of the biomass starting material (Technology Report, Chem. & Eng. News, pp. 24, Feb. 23, 1976). Fermentations leading to the production of ethanol are even less efficient (50% cellulose in wood.times.50% conversion to glucose.times.95% yield of ethanol.times.67% carbon recovery=16% yield) (Goldstein, I. S., Chem. & Eng. News, pp. 4, Dec. 6, 1976). The theoretical limit of energy conversion by ethanol production is 67% due to the loss of one/third of the available carbon as carbon dioxide gas.
Pyrolyric processes also tend to be inefficient. For example, the pyrolytic conversion of biomass to water gas (CO.sub.2 +H.sub.2 CO+H.sub.2 O) results in a large loss of the intrinsic caloric value of the starting material due to the loss of 30-50% of carbon as carbon dioxide (Goldstein, I. S., Science 189:847 (1975)). Pyrochemical processes which convert biomass into crude fuel oil lose carbon as char and gases (Appell, H. R., et. al., Bureau of Mines R.I. #7560, U.S. Department of the Interior, Washington, D.C., (1971); Appell, H. R., et. al., Bureau of Mines R.I. #8013, U.S. Department of the Interior, Washington, D.C., (1975)). As a result, such processes typically result in only about an 80% conversion of biomass carbon. Because of these and other factors, quantitative yields of suitable liquid fuels have not been obtained using either fermentations or pyrolytic reactions.
Pyrolytic processes require dry feedstocks (Soltes, E. J., ACS Symposium Series #376, chapter 1, (Soltes, E. J. and Milne, T. A., eds.) American Chemical Society, Washington, D.C., (1988)). As a result, it is impractical to use such processes with those sources of biomass that have a high moisture content. For example, it would not be practical to use aquatic plants in most pyrolytic reactions.
There is an increasing need for a variety of fuels (especially conventional liquid fuels) and for new sources of conventional chemicals and chemical feedstocks (Goldstein, I. S., pp. 4, Chem. & Eng. News, Dec. 6, 1976; Sarkaren, K. V., Science 191:773 (1976)). Processes capable of producing such products in high efficiency from readily available sources of biomass would clearly be of great value.
II. Conversion of Polyhydric Alcohols to Iodoalkanes and Hydrocarbons
The cellulose and hemicellulose components of biomas, could potentially provide the starting material for a chemical pathway for the production of hydrocarbon fuels and chemical feedstocks. Efficient methods for converting these components into polyhydric alcohols have been reported in the literature (see Sharkov, V. I., Angew. Chem. I.E.E. 2:405 (1963); see also Creighton, H. J., Trans. Electrochem. Soc. 75:289 (1939)). Although methods are available for reacting polyhydric alcohols with hydriodic acid to produce iodoalkanes these methods have been inefficient either because of poor rates of conversion or because they have required the use of massive quantities of reagents.
Historically, the chemical conversion of reactants to 2-iodohexane has been used as a means of proving the structure of sugars. Typically, polyhydric alcohols such as sorbitol are reacted with a very large excess of hydriodic acid (e.g. at an 85/1 mole ratio), in a sealed glass capillary, at 135.degree. C. and at low pressure to give 2-iodohexane (Mitchell, H. K. et. al., J. Amer. Chem. Soc. 60:2723 (1938)). Under these conditions, five hydroxy groups are reduced, and one hydroxy group undergoes substitution. The reaction is conducted on an analytical scale, and a titration is then performed to determine the amount of iodine liberated by the reduction reaction. This reaction is the basis for Zeisel's test for alkoxy groups on sugars.
When the Zeisel type reaction is performed on a larger scale, with a lower ratio of HI to polyhydric alcohol, only small amounts of iodoalkane are produced. The reaction stops due to mechanical problems associated with phase separation caused by the presence of both a large amount of iodine and, perhaps, polyiodo intermediates. Apparently, these by-products are poorly soluble in a simple aqueous system.
A Japanese patent describes the reduction of sorbitol with HI, but 2-iodohexane was recovered in only a 22% yield (Nakamura, Y., Jap. Pat. No. 78,144,506, (1978); Chem. Abstr. 91:19910.times.(1979)). Nakamura used a 1/1 mole ratio of reactants in an acetic acid solvent containing H.sub.2 and chloroplatinate. The reaction was performed at 110.degree. C. and 710 psi for 2.5 hours.
An article published in 1890 reported that sorbitol reacts with aqueous HI and red phosphorous to produce 2-iodohexane in 95% yield in a reaction performed on a 0.3 mole scale (Vincent, C., Compt. Rend 109:677 (1890); Beil. 59:533 (1918)). By-product I.sub.2 is consumed by the phosphorous during the reaction. This result demonstrates that sorbitol can be converted to iodohexane in high yield if I.sub.2 reacts quickly enough so that it does not interfere with the reduction reaction. Unfortunately, the solid red phosphorous creates mechanical problems in the reaction due to the presence of heterogeneous phases. Also, the rate of I.sub.2 conversion is limited by the surface area of the solid particles.
Another early reference reported that, when sorbitol and mannitol are reacted with HI and red phosphorous in a sealed tube at 250.degree. C. for 5 hours, small amounts of high molecular weight hydrocarbons are produced in addition to iodohexane (Willstatter, R., et. al., Chemische Ber. 55b:2637 (1922); Chem. Abstr. 17:982 (1923)). These hydrocarbons are similar in composition to those produced in the same way from cellulose, lignin, or other carbohydrates.
Ideally, the efficient conversion of polyhydric alcohols to iodoalkanes and hydrocarbons would be accomplished by homogeneous chemical agents that rapidly reduce I.sub.2. This would provide a key reaction in pathways for the conversion of sorbitol or xylitol derived from biomass into hydrocarbon fuels and chemical feedstocks.